High purity water is required for many purposes, including use in pharmaceuticals, medicine and biology. Many industrial manufacturing processes require the use of ultra high pure water either as a direct process fluid or as the major component of a liquid product. This is particularly true of the pharmaceutical, electronic and electrical utilities industries. Purity of water used in the pharmaceutical industry is clearly required because of the health involvement in the final product. Greater and greater purity of water in the electronics industry is required due to the continually greater miniaturization in the manufacture of electronic devices on semiconductor substrates, such as single crystal silicon wafers. Impurities on the substrates in the region of the electronic device formation cause defects in the devices formed, which may considerably lower the yield of good products of the manufacturing process as well as affecting the long term reliability of the product manufactured.
For example, heavy metal impurities, such as copper, nickel, chromium and especially iron, in the crystal silicon wafers have adverse effects on the electrical properties thereof and on the yields of the integrated circuits formed therefrom. The actual effects of the heavy metal impurities on silicon electrical properties are varied, depending on the form that the impurity takes in the matrix of the silicon crystal. Metallic impurities which remain dissolved within the silicon matrix create mid-band gap trap centers which increase generation rate and ultimately result in increased current leakage of diffused junctions. In addition, heavy metal impurities can agglomerate and form separate and distinct metal-silicide precipitates. The effect becomes even more pronounced as the products become miniaturized, especially in the manufacture of ultra large scale integrated circuits.
Water is required in large quantities for rinsing semi-conductor products after many steps in the production process and/or to remove contaminants from the silicon wafers. However, when water contains contaminants, especially metals, they will deposit on the silicon wafers, creating the aforementioned defects. To minimize contamination, the water utilized should be as pure as possible, and thus contain as little metal contaminants as possible. Therefore, ultrapure water is required in the processes.
It is important that once the water of ultrapure quantity is obtained, it remains ultrapure and does not become contaminated with metal contaminants. Unfortunately, quite often, the quality of the water deteriorates over time. This can occur in a variety of ways. To illustrate this, it is necessary to digress and generally outline the production of ultrapure water.
In the conventional production of ultrapure water, the starting water is passed through a pretreatment apparatus and a primary pure water production apparatus. The pretreatment apparatus and the primary water apparatus are ordinarily installed in a building apart from a factory building in which semi-conductor production apparatuses, for example, are installed. The primary pure water is introduced into a high purity water tank installed in a factory building via a primary pure water pipe.
The water stored in the water tank is passed through a subsystem wherein the water is treated and converted to ultrapure water. This ultrapure water is introduced into the semiconductor production apparatus via an ultrapure water pipe. The ultrapure water is continuously recirculated through a point just before the semiconductor production apparatus and in returned to the high purity tank via a return pipe. Thus, a loop is formed between the high purity water tank, the treatment subsystem, the ultrapure water pipe and return pipe, and the ultrapure water is circulated constantly through the loop.
It is known, however, in the above-described conventional arrangement, that the quality of the ultrapure water is reduced in its purity when it stops flowing and is stagnant. Contaminants, such as inorganic salts, especially metal containing inorganic salts contained in the pipe, etc. dissolve in the ultrapure water at the portion of the pipe, etc. contacting the ultrapure water. Therefore, the ultrapure water has to be circulated constantly in the loop to prevent deterioration of the purity of the ultrapure water.
However, the subsystem and the semiconductor production apparatuses are installed relatively far from each other. This arrangement necessitates the use of a long pipe for feeding the ultrapure water from the subsystem to the semiconductor apparatuses; the loop of the ultrapure water pipe reaches 100-500 meters in some cases. When ultrapure water is passed through a long pipe, the purity of the ultrapure water is reduced owing to, for example, the dissolution of impurities such as metal contaminants in the pipe. Further with the adoption of more complex semiconductor production steps and nondiversified semiconductor production apparatuses, the length of the ultrapure water pipe has become larger necessarily, which has resulted in a reduction in the quality of ultrapure water. In view of the trend towards higher density integration of circuitry used in semiconductors, it is important that the quality of the water introduced remains high. Therefore, the ultrapure water at its point of use needs to be monitored continuously to verity that the water is of sufficient quality. For example, in view of the adverse effects of the metal contaminants, it is important to monitor the water and verify that the concentration of the metal contaminants does not exceed tolerable levels.
Unfortunately, an adequate method of monitoring remains a challenge, especially at very low levels. The types of techniques utilized to date include, inter alia, Scanning Electron Microscopy, (SEM), particle counting tools, GC, FTIR, HPLC, ion chromatography, IR, UV, inductively coupled plasma (ICP), and atomic absorption. While SEM has become less expensive to implement and has excellent sensitivity, it requires effort, time and reasonable statistical models for use. Particle counting tools such as the Unizak model ERC 9320 counts particles from 0.3 to 0.05 micrometers in size in ultra pure water, but the results are not easily reproducible or interpretable. The other analytic methods, such as atomic absorption (AA), inductively-coupled plasma/mass spectrometry (ICP/MS) or X-ray fluorescence are impractical to implement as rapid, point of use monitors.
Therefore, it becomes important to find a methodology which is accurate, sensitive to metal or cation contaminants, provide reproducible results and can be utilized at point of use.
Moreover, it becomes imperative to find a methodology which would monitor the concentration of these cation contaminants at reasonable costs. To date, the only methodology available is the use of sensors for each individual cation that is suspected of being present in the system that is being monitored. This is quite expensive in and of itself. However, as the number of metals that are being monitored increases, the amount of equipment required also increases, and concomitantly therewith, so does the expense. Therefore, a methodology is desired that would monitor trace concentrations of metals more economically.
The present invention accomplishes these objectives and has these advantages.
More specifically, the present invention provides a way to monitor in real time at point of use the concentrations of a wide variety of trace impurities in the water. In utilizing the method of the present invention, it is possible to determine the amount of trace impurities in the water and to assess whether the water is of a sufficient purity for it to be utilized for its intended purpose. Furthermore the costs involved utilizing this method are significantly less than those associated with detecting trace cations heretofore.